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Chapter 4 Atomic Structure 4.1.) Early Models of the Atom Atom – the smallest particle of an element that retains its identity in a chemical reaction - Democritus, Greek philosopher, believed that atoms were indivisible and indestructible o No experimental support - Robert Dalton’s theory, using experimental methods, proposed: o All elements are composed of tiny indivisible particles called atoms o Atoms of the same element are identical o Atoms of different elements can physically mix or chemically combine in simple whole-number ratios to form compounds o Chemical reactions occur when atoms are separated, joined, or rearranged, but they are never changed into another element - The radii of most atoms is between 5 x 10-11 m to 2 x 10-10 m o Individual atoms can be observed with instruments like a scanning tunnel microscope 4.2.) Structure of the Nuclear Atom - Atoms are divisible into three kinds of fundamental particles called electrons, protons, and neutrons - J.J. Thomson discovered the electron, a negatively charged subatomic particle, by deflecting a cathode ray through magnets or electrically charged plates o Thomson knew that opposite charges attract and like charges repel, so he sent a stream of negatively charged particles (originally called corpuscles) at high speed. The particles were deflected toward the positive plate - Robert Millikan found the charge on an electron to be one negative unit. Its mass is 1/1840th the mass of a hydrogen atom - Eugene Goldstein named the positively charged subatomic particles protons, with a mass about 1840 times that of an electron - James Chadwick confirmed the existence of the neutron as a particle with no charge but a mass slightly larger than a proton’s The Atomic Nucleus - Thomson’s atomic model, the “plum-pudding model,” had the electrons stuck into a lump of positive charge - Ernest Rutherford tested atomic structure theory by directing a narrow beam of alpha particles (helium atoms that have lost their electrons) at a thin sheet of gold foil. The particles should have been deflected but most were not. o Rutherford proposed that the atom is mostly empty space with most of the mass concentrated in the nucleus - Known as the nuclear model 4.3.) Distinguishing Among Atoms Elements are different because they contain different numbers of protons - Atomic Number: the number of protons in the nucleus of that element o In electrically neutral atoms, the number of protons equals the number of electrons - Mass number is the total number of protons and neutrons in an atom o The number of neutrons in an atom is the difference between the mass number and the atomic number o Shorthand notation uses the chemical symbol of the element with the atomic number written as the subscript and the mass number as the superscript 197 79 Au o Atoms can also be referred to using the mass number and the element name; i.e. gold-197 Isotopes- atoms that have the same number of protons but different numbers of neutrons - Because isotopes have different numbers of neutrons, they also have different mass numbers o Three known isotopes of hydrogen: - Hydrogen (hydrogen-1) has one proton and no neutrons - Deuterium (hydrogen-2) has one proton and one neutron - Tritium (hydrogen-3) has one proton and two neutrons - Atomic Mass is determined by a mass spectrometer o Using carbon-12 as the standard, an atomic mass unit (amu) is defined as one twelfth of the mass of a carbon-12 atom o In nature, most elements occur as a mixture of two or more isotopes o Atomic mass is the average mass of the atoms in a naturally occurring sample of the element - To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products The Periodic Table – A Preview - A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties - A periodic table allow you to easily compare the properties of one element (or group of elements) to another element (or group of elements) o Each horizontal row is called a period - Seven periods with repeating patterns in each period - Each vertical column is called a group or family, having similar chemical and physical properties